In digital radio communication system, CDMA (Code Division Multiple Access) as a multiple access method takes benefits of the orthogonality between spreading codes, resulting in no interference due to cross correlation between spreading codes, thus, a plurality of spreading codes can be multiplexed and transmitted.
However, because there is a multipath in real propagation environments, the orthogonality between spreading codes is no longer hold and an interference is generated when multiplexing (code multiplexing) and transmitting a plurality of spreading codes. In CDMA, RAKE combining is employed and effective advantages were shown when using RAKE combining in which a plurality of paths are combined in multipath propagation environments encountered a large interference deterioration.
Several methods to cancel multipath interference have been proposed. As one of those methods, the method of Karimi (“EFFICIENT MULTI-RATE MULTI-USER DETECTION FOR THE ASYNCHRONOUS WCDMA UPLINK”, H. R. Karimi, VTC'99, pp. 593–597: Joint Detection: JD). Such a joint detection method will be explained using FIG. 1.
A radio signal is received in radio receiving section 52 via antenna 51. In radio receiving section 52, predetermined radio reception processing (such as, down conversion, A/D conversion, etc.) are performed on the received signal, and the signal being subjected to radio reception processing is outputted to RAKE combining section 53 and correlation processing section 54.
In correlation processing section 54, a correlation processing between a known signal (midamble) and the signal being subjected to radio reception processing is carried out. The signal after correlation processing (correlation result) is outputted to delay profile generating section 55. A delay profile is generated in delay profile generating section 55 based on the correlation result. This delay profile is outputted to RAKE combining section 53.
In RAKE combining section 53, RAKE combining is carried out based on the delay profile, and the result of RAKE combining is outputted to JD section 56. Joint detection processing is performed on the signal being subjected to RAKE combining in JD section 56 in accordance with maximum delay time. Receiving signals of all codes are outputted after the joint detection processing.
In such a joint detection processing, a matrix A is generated by calculating the convolution between the delay profile and spreading codes, then, the cross correlation AHA of the matrix A is calculated, finally, from the expression (AHA x=b), in which the cross correlation AHA is multiplied by the transmission symbol x and b is obtained after RAKE combining multiplication result, the interference caused due to cross correlation between spreading codes is cancelled and keeping only transmission symbol x given in the above expression, then the receiving signals of all codes are outputted.
The amount of calculations to generate the matrix A in the joint detection processing is given by (NQ+W−1)×(KN) assuming that N is the number of transmission symbols, Q is the spreading sequence, W is the delay time window (expressed in CDMA chip time unit), and K is the number of the transmitting multicode. Moreover, the amount of calculating the cross correlation AHA becomes KN×KN which is extremely large amount. In this case, the influence of the delay time window W is extremely large.
In conventional radio reception method, the delay time window W is set to include the delay signal of maximum delay time. In other words, as can be explained using FIG. 4, the delay time window W is set to include all the 3 paths in which |α2|2 which has the greatest delay time is included. Such a window is similar to the window used in RAKE combining.
On the other hand, the advantages of using RAKE combining are difficult to be achieved if the window width W of both RAKE combining and joint detection is simply set to small value so that only one of the multipath may be included, in order to decrease the amount of calculation.